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Overview
========
The errata is worked around in the kernel without requiring toolchain
modifications. It is triggered the JMPL/RETT return from trap instruction
sequence never generated by the compiler and. There are also other
conditions that must must be true to trigger the errata, for example the
instruction that the trap returns to has to be a JMPL instruction. The
errata can only be triggered if certain data is corrected by ECC
(inflicted by radiation), thus it can not be triggered under normal
operation. For more information see:
www.gaisler.com/notes
Affected RTEMS target BSPs:
* GR712RC
* UT699
* UT700/699E
The work around is enabled by defining __FIX_LEON3_TN0018 at build time.
After applying the following GCC patch, GCC will set the define when
compiling for an affected multilib:
* GR712RC (-mcpu=leon3 -mfix-gr712rc)
* UT700/UT699E (-mpcu=leon3 -mfix-ut700)
* UT699 (-mcpu=leon -mfix-ut699)
When building for another multilib and TN0018 is still required, it
is possible to enable it on the RTEMS kernel configure line using the
TARGET_CFLAGS (-D__FIX_LEON3FT_TN0018) or other by other means.
The following GCC patch sets __FIX_LEON3FT_TN0018 for the affected RTEMS
multilibs:
---------
diff --git a/gcc/config/sparc/rtemself.h b/gcc/config/sparc/rtemself.h
index 6570590..ddec98c 100644
--- a/gcc/config/sparc/rtemself.h
+++ b/gcc/config/sparc/rtemself.h
@@ -33,6 +33,8 @@
builtin_assert ("system=rtems"); \
if (sparc_fix_b2bst) \
builtin_define ("__FIX_LEON3FT_B2BST"); \
+ if (sparc_fix_gr712rc || sparc_fix_ut700 || sparc_fix_ut699) \
+ builtin_define ("__FIX_LEON3FT_TN0018"); \
} \
while (0)
---------
Workaround Implementation
=========================
In general there are two approaches that the workaround uses:
A) avoid ECC restarting the RETT instruction
B) avoid returning from trap to a JMPL instruction
Where A) comes at a higher performance cost than B), so B) is used
where posssible. B) can be achived for certain returns from trap
handlers if trap entry is controlled by assembly, such as system calls.
A)
A special JMPL/RETT sequence where instruction cache is disabled
temporarily to avoid RETT containing ECC errors, and reading of RETT
source registers to "clean" them from incorrect ECC just before RETT
is executed.
B)
The work around prevents JMPL after system calls (TA instruction) and
modifies assembly code on return from traps jumping back to application
code. Note that for some traps the trapped instruction is always
re-executed and can therefore not trigger the errata, for example the
SAVE instruction causing window overflow or an float instruction causing
FPU disabled trap.
RTEMS SPARC traps workaround implementation:
NAME NOTE TRAP COMMENT
* window overflow 1 - 0x05 always returns to a SAVE
* window underflow 1 - 0x06 always returns to a RESTORE
* interrupt traps 2 - 0x10..1f special rett sequence workaround
* syscall 3 - 0x80 shutdown system - never returns
* ABI flush windows 2 - 0x83 special rett sequence workaround
* syscall_irqdis 4 - 0x89
* syscall_irqen 4 - 0x8A
* syscall_irqdis_fp 1 - 0x8B always jumps back to FP instruction
* syscall_lazy_fp_switch 5 - 0x04 A) jumps back to FP instruction, or to
B) _Internal_error() starting with SAVE
Notes:
1) no workaround needed because trap always returns to non-JMPL instruction
2) workaround implemented by special rett sequence
3) no workaround needed because system call never returns
4) workaround implemented by inserting NOP in system call generation. Thus
fall into 1) when workaround is enabled and no trap handler fix needed.
5) trap handler branches into both 1) and returning to _Internal_error()
which starts with a SAVE and besides since it shuts down the system that
RETT should never be in cache (only executed once) so fix not necessary
in this case.
Any custom trap handlers may also have to be updated. To simplify that,
helper work around assembly code in macros are available in a separate
include file <libcpu/grlib-tn-0018.h>.
Close #4155.
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This patch is a part of the BSP source reorganization.
Update #3285.
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This patch adds NOP instructions to prevent instruction sequences
that are sensitive to the LEON3FT B2BST errata. See GRLIB-TN-0009:
"LEON3FT Stale Cache Entry After Store with Data Tag Parity Error"
for more information.
The sequences are only modified if __FIX_LEON3FT_B2BST is defined.
The patch works in conjunction with the -mfix-ut700, -mfix-gr712rc,
and -mfix-ut699 GCC flags that prevents the sensitive sequences from
being generated.
Update #3057.
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- All references of %0 changed to %g0
- 'call label,0' changed to 'call label'. According to the sparc
specification call does not take any registers
- '.seg "text"' changed to '.section ".text"'
- the synonym stub is replaced with stb
- the synonym stuh is replaced with sth
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The window underflow trap handler used %i5 which destroyed the %o5 of
the calling context. Bug introduced by
0d3b5d47429effb350448d9e9123a67db722109f.
Go back to the pre 0d3b5d47429effb350448d9e9123a67db722109f behaviour
and use the two unused instructions in the trap vector to optimize a
bit.
Update #2651.
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Save five instructions on underflow handling.
By using an optimized trap entry we can move instructions from
the window underflow function into the trap entry vector. By
setting WIM=0 and using RESTORE it is possible to move the
new WIM register content from the trapped window into the
to-be-restored register window. It is then possible to avoid
the WIM write delay.
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By using a optimized trap entry we can move instructions from
the window overflow function into the trap entry vector. By
using the saved locals instead of g1 we don't need to save
that register temporarily. Also spead out non store instructions
inbetween stores to use the write buffer better.
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I see no need for waiting the 3 instruction delay for wim to be
written in this case, since the STD after does not depend on WIM
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Script does what is expected and tries to do it as
smartly as possible.
+ remove occurrences of two blank comment lines
next to each other after Id string line removed.
+ remove entire comment blocks which only exited to
contain CVS Ids
+ If the processing left a blank line at the top of
a file, it was removed.
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* reg_win/window.S: Include <rtems/asm.h> instead of <asm.h>.
* syscall/syscall.S: Include <rtems/asm.h> instead of <asm.h>.
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.s files to .S in conformance with GNU conventions. This is a
minor step along the way to supporting automake.
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